^ Also, Spock knows that Nero arrived some time ahead of him and has been doing God-knows-what with the timeline. He has no way of knowing how much of the history he remembers has been altered. Then, immediately after Vulcan goes kabloom, Kirk comes barreling into Spock's cave. Spock's initial assumption, that somehow Captain Kirk has come to rescue him ("How did you find me?"), is pretty reasonable under the circumstances.